These Best Practices for Olive Oil Processors are based on information presented at the Master Milling Short Course held each October at the UC Davis Olive Center.
Choose the right olive varieties for the right growing conditions. Olives grown under irrigation in warmer climates allow the fruit to fully develop, which often leads to sweeter, milder oils. Olives grown under deficit irrigation in cooler climates tend to produce oils that are more robust, with more bitterness and pungency. Generally, it is challenging to grow cool-climate varieties in a desert climate or to produce a sweet and mild oil in a cool climate. In general, increasing irrigation will reduce bitterness and pungency, as will harvesting later in the season.
Harvest healthy fruit. The best quality olive oil comes from healthy trees, with fruit that has not been damaged by pests such as olive fruit fly, diseases such as olive knot, or adverse environmental conditions such as drought and frost. To produce high-quality extra virgin olive oil, do not harvest fruit from the ground.
Time the harvest for maximum quality and extraction. To achieve an optimal balance between oil quality and extraction efficiency, harvest the orchard as soon as the oil content has reached its maximum weight, and preferably when fruit moisture is between 50 and 55 percent. Measure oil and moisture content either with a near-infrared (NIR) instrument or by drying olives in a dehydrator or an oven. Collect and weigh a 100-olive sample from the area to be harvested. If using an oven, dry for one hour at 266 °F (130 °C), then turn the samples over and dry in the oven for an additional hour. Let cool and then weigh the olives again. Compare this weight to the fresh sample weight to determine moisture content. Calculate oil content by performing the same steps weekly as harvest time approaches, and harvest when the dry sample weight no longer increases over the previous week’s weight.
Use the maturity index to estimate flavor profile when evaluating harvest timing. The maturity index provides information that may be useful in evaluating harvest timing to achieve a consistent style of oil from year to year. Greener fruit usually offer oils with typical green-fruit character (such as fresh-cut grass, leafy, green fruit, and herbaceous flavors) as well as elevated bitterness and pungency. Riper fruit generally provides ripe-fruit character (such as nutty, buttery and tropical flavors) with reduced bitterness and pungency. A small difference in fruit maturity can have a great influence on the fruity component of the oil, but it does not reliably correlate with oil accumulation or polyphenol content in the fruit.
Choose an efficient harvest method that preserves quality. Choose harvest equipment that will minimize the amount of time that it takes for the fruit to get to the processing plant and that minimizes fruit damage. Shakers do not perform at a high rate of removal with green fruit, so the shaker may also need some manual assistance with tree-beating or hand-harvesting, or with the use of a loosening agent. Many mechanical harvesters can deliver fruit in good condition, with the exception of when laborers are beating trees with rakes and stepping on fruit on the tarps. The more the fruit is damaged the more critical it is that the fruit be processed quickly to achieve a high quality olive oil.
Minimize foreign material in harvest containers. Keep leaves, rocks, twigs and other “material other than olives” (MOO) below 2.5 percent in harvest containers. Too much MOO usually means that there are problems in the orchard such as the fans on the harvest equipment not working correctly or the use of excessive levels of fruit-loosening agent in the orchard. Although equipment at the processing plant usually will remove much of the MOO, MOO can have a negative impact on the quality of the oil and may damage processing equipment. While adding leaves to the crusher can temporarily increase the green color and polyphenols of olive oil, the leaves also add an unpleasant off-flavor and astringency to the oil.
Transport fruit quickly using appropriate containers. Post-harvest concerns include fruit damage, transport time, and the ambient temperature, as it is extremely important to avoid fruit fermentations. These factors are not very important if the fruit is processed within about four hours of harvest, but become critical when processing lags beyond four hours. The longer it takes to process the olives the more critical it will be to have less fruit damage, smaller-sized and well-aerated containers, and lower outside temperatures. The worst containers for olive transport are those that do not allow proper ventilation, particularly bags.
Ensure that the fruit is properly cleaned before crushing. The fruit needs to undergo a cleaning process to eliminate the MOO and presence of dust. Cleaning systems blow away leaves, dust and other MOO using a fan. The fruit can then be washed if required, but washing may also reduce oil extractability and the level of polyphenols of the oil.
Evaluate the fruit condition to decide how to process. Prior to crushing, evaluate maturity, moisture and variety of the fruit to determine the most appropriate strategy for paste preparation. The fruit condition will also determine if the operator should use processing aids such as talcum powder and enzymes to improve oil extractability and decanter working-capacity.
Choose the correct grid size for proper crushing degree. Low-maturity fruit will characteristically require a fine crushing degree, and therefore a small grid size in the crusher, to maximize oil extractability. Riper fruit with softer cell tissues, as well as high-moisture fruit, are best prepared with larger grid sizes (6 - 7mm) to maximize extraction efficiency. De-pitting the olives prior to crushing can modify the aromatic profile and phenolic composition of the oil, however, this practice also reduces extraction efficiency.
Adequately adjust the malaxation parameters. Keep malaxation times within 75 minutes to allow for a steady kneading of the paste and a slow release of the oil. Lower malaxation temperatures tend to generate more aromatic and complex oils but also reduce extraction efficiency. Conversely, higher malaxation times maximize paste extractability but reduce the complexity of the oil. Temperatures of 80 – 86 °F (27 – 30 °C) represent a good compromise in achieving high oil quality and extraction efficiency. Frequently inspect the paste in the malaxer to look for the presence of free oil being released from the paste. Malaxation with nitrogen blanketing or under vacuum can marginally improve oil quality.
Consider the use of processing aids when required. Talcum powder and pectinase enzymes can help in improving the physical condition of the paste for a better oil extractability. Plan to add talcum powder to the paste in the malaxer if the fruit that will be processed is excessively wet (moisture > 58%) or if the paste looks emulsified in the malaxer. Also consider adding pectinase enzymes to the paste when the fruit is of low maturity (green or turning-color). These products usually have a synergetic action when used together and can be added at the same time in the malaxer.
Adjust the decanter speed and monitor oil losses in the pomace. When processing the paste, ensure that the oil emerging from the decanter is not too “dirty” with paste as this could mean that the paste feeding-speed is excessively high or that the decanter plates are excessively open. In the case of a two-phase decanter, consider injecting up to 5 - 6% of water to reduce the chances of losing oil in the waste. Check the waste regularly to determine the extent of oil losses, as well as whether paste-preparation decisions were correct and whether to slow down the feeding rate of paste into the decanter.
Continually inspect the condition of the oil coming out from the vertical centrifuge. Check the oil coming out of the vertical centrifuge to ensure that it appears clean with a milky aspect. If the oil has a shiny aspect it may mean that there are excessive temperatures in the malaxer, or that excessively warm water was added into the decanter or the vertical centrifuge. Visually ensure that the vertical centrifuge has no oil coming out from the water outlet, or water from the oil outlet. It is safe to add to the centrifuge a small amount of water with a similar temperature to the oil, or 2 – 4 °F higher, to improve the cleaning operation.
Let the oil settle before storing. Allow the oil to rest in settling tanks to separate the oil from fine water droplets and particles in suspension. The settling process will also help release most of the air bubbles contained in the oil. Ensure that the settling tanks are drained regularly to remove sediments and water. After a minimum of 16 hours of settling, take a sample of the oil and, if possible, carry out an in-house sensory analysis as well as testing of free fatty acids and peroxide value. Based on this sensory and chemical analysis, decide on the final destination of the oil in the tank storage facility.
Store the oil properly to maximize the shelf life of the product. Utilize stainless steel tanks, nitrogen blanketing and temperature control in the storage room to minimize oxidation processes in the oil. The oil is best kept at temperatures ranging between 59 °F and 66 °F (16 °C – 19 °C.) Ensure that the storage tanks are drained regularly to remove water and sediments to minimize hydrolytic reactions in the oil. Once the oil has fully settled (approximately 45 days after processing depending on room temperature), move the oil to a new clean tank to remove the “mud” normally stuck to the bottom of the tank. Finally, send oil samples to a laboratory for chemical and sensory testing to ensure that the oil complies with trade standards.
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